Recently, non-aqueous electrolyte secondary batteries have been actively researched and developed for use of portable devices, hybrid cars, electric cars, or storage of electricity for home use. The non-aqueous electrolyte secondary battery used in these fields requires a high safety. In order to satisfy the requirement, a battery has been developed which uses a titanium compound as a negative electrode active material. The battery using the titanium compound as the negative electrode active material, however, has a problem in which an electrolytic solution is reacted with the negative electrode active material to generate gas, whereby a cycle stability is reduced.
In order to consume the gas generated in the battery, for example, Patent Document 1 has developed a technique in which a gas-removing agent containing an organic polymer having a double bound and a catalyst is introduced into an outside of an active battery volume part, whereby the gas generated in the battery during operation can be consumed. The technique, however, is applied to an alkaline dry cell, and the document does not refer to the effects on the non-aqueous electrolyte secondary battery at all. In the non-aqueous electrolyte secondary battery, the performance is remarkably deteriorated by moisture, and thus the incorporation of the gas-removing agent into the outside of the active battery volume part is not suitable for the non-aqueous electrolyte secondary battery.
Patent Document 2 describes that when spinel type lithium manganate is used as a positive electrode active material, the amount of gas generated in the battery can be reduced by adding lithium cobaltate or lithium nickelate. In such a mixed positive electrode, however, the different positive electrode active materials, i.e., the spinel type lithium manganate, and the lithium cobaltate or the lithium nickelate, are exposed to the same potential environment during charge and discharge, and the latter materials are deteriorated, thus resulting in, sometimes, reduction of a capacity retention in a cycle test for a long period of time. The improvement thereof is necessary.
Patent Documents 3 to 5 disclose that positive electrodes are formed from a plurality of layers of a positive electrode active material layer containing a spinel type lithium manganate and a positive electrode active material layer containing lithium cobaltate. It can be considered that these techniques have a certain effect to inhibit the gas generation, but the cycle characteristics are reduced, which may be caused by the lithium cobaltate, during a cycle test, and thus these techniques must be improved.